Some Group III or IV metal-containing nitrides such as GaN, AlN, TiN, HfN, Si3N4, and BN can be useful as superconducting materials, semiconducting materiais, optoelectronic materials, ultrahard ceramic materials, or solid lubricants, and have widely been used as industrial materials.
These nitrides are prepared as powder materials by direct nitrization of metals or by pyrolysis of precursor compounds. Most of such nitrides are therefore formed into sintered products for use in a variety of industrial materials. Recently, the nitrides have also been prepared as thin films or oriented films close to single crystal by chemical vapor deposition or physical vapor deposition such as sputtering in order to perform their useful functions as optoelectronic materials or coating materials.
Many of the Group IV metal-containing nitrides form specific layer structured crystals, which are used as solid lubricants and useful intermediates for ZrN or HfN nitride ceramics.
The Group IV metal-containing nitrides also include ternary transition metal nitride halides having a chemical composition represented by the formula: MNX (M═Zr, Hf, X═Cl, Br, I). Such metal nitride halides are known to have two polymorphic forms called α and β forms.
The α form has the FeOCl-type layer structure, and the β form has a SmSI-type layer structure. In particular, the β form of the MNX is an important nitride, because it becomes a superconductor with a relatively high critical temperature (Tc) when alkali metal is intercalated (inserted) between its crystal layers, and electrons are doped into its nitride layer. For example, β-ZrNCl and β-HfNCl intercalated with lithium become superconductors with Tcs of about 14 K and 25.5 K, respectively, which have been noted as new high-Tc superconductors.
Ohashi et al. have succeeded in synthesizing β-ZrNCl powder in high yield by the reaction of metallic zirconium or zirconium hydride with ammonium chloride vapor at high temperature (J. Solid State Chem., 75, 99 (1988)).
Ohashi et al. have also succeeded in obtaining a well crystallized sample by a chemical transport of the as prepared powder sample in a vacuum-sealed quartz tube (J. Solid State Chem., 77, 342 (1988)). In the chemical transport method, a small amount of ammonium halide such as ammonium chloride as a transporting agent is added into the quartz tube for the chemical transportation of β-MNX.
Ohashi et al. have provided a breakthrough for the β-MNX research by developing such a chemical transport method for purification and crystallization of β-MNX layer structured crystal. However, such a method can only produce thin plate-shaped polycrystalline samples.
Yamanaka et al. have succeeded in synthesizing the β-form crystal phase of all the compositions of the MNX (M═Zr, Hf, X═Cl, Br, I) and reported that all the compositions can show superconductivity by intercalation of alkali metal (Inorg. Chem., 39, 806 (2000)). In the reported process, the α-form powder sample was converted into the β-form by compressing it under a high pressure of 1 to 5 GPa. However, the resulting crystal was not a single crystal but a polycrystalline powder.
The β-MNX layer structured nitrides are semiconductors with a wide band gap, which are not only potential host materials for superconductors but also potential optoelectronic materials.
The Group III or IV metal-containing nitrides are potential materials for semiconductors, optoelectronics, or superconductors. Such nitrides are, however, mostly prepared as powder materials; even the chemical transport method can only provide very thin and soft polycrystals. These nitrides should be prepared as well developed single crystals in order for practical use as industrial materials. However, the production of such single crystals has been considered to be very difficult, and no method has been reported on such production yet.
It is therefore an object of the present invention to provide such nitride single crystals and the methods for preparing the same.